Surface-coated cutting tools having single-layer or multi-layer hard coatings of TiC, TiN, TiCN, Al2O3, etc. on the surfaces of cemented carbide substrates have been conventionally used to cut works. When such surface-coated tool is used to cut difficult-to-cut alloys such as Ni-based heat-resistant alloys such as Inconel (registered trademark), heat-resistant stainless steel, etc., cutting edges are chipped due to the work hardening of difficult-to-cut alloys by cutting; hard coatings are peeled by the repeated detachment of difficult-to-cut alloys seized to the cutting edges; and the substrate matrix are destroyed by the peeling of the hard coatings. Therefore, cutting tools having hard coatings excellent not only in wear resistance but also in chipping resistance are desired.
JP 9-262705 A discloses a coated cutting tool comprising a first layer constituted by a reacted solid solution of TiN having a granular crystal structure and substrate components, and a second layer constituted by a reacted solid solution of columnar crystal TiCN and substrate components, on a WC-based cemented carbide substrate. It has been found, however, that because the TiN film is formed at 920° C. using a starting material gas comprising a TiCl4 gas, a N2 gas, and a H2 gas in JP 9-262705 A, the starting material gas is excessively reacted with the substrate, excessively diffusing W in the substrate, resulting in a brittle layer under the first layer, and thus poor chipping resistance.
Japanese Patent 3503658 discloses a coated cutting tool comprising a lower layer of TiC or TiCN, an intermediate layer of columnar crystal TiCN, and an upper layer of TiC or TiCNO on a WC-based cemented carbide substrate, any of the lower layer, the intermediate layer and the upper layer containing 0.01-6% by mass of Co coming from the substrate. However, because the lower layer, the intermediate layer and the upper layer contain too much Co, the hard coating has low hardness and thus insufficient wear resistance. It has also been found that because Co and W are excessively diffused from the substrate, a brittle layer is formed, resulting in poor chipping resistance.
JP 2011-36988 A discloses a coated cemented carbide tool comprising a first layer (TiC film) and a second layer (columnar crystal TiCN film) formed on a WC-based cemented carbide substrate, the amounts CrL1, CoL1 of Cr and Co diffused from the substrate to the first layer meeting the conditions of 0.1≦CrL1≦0.6 by % by mass, and 0.6≦CrL1/CoL1≦1.6. It has been found, however, that because the first layer of TiC is formed at 930° C. or higher using a starting material gas comprising a TiCl4 gas, a CH4 gas, and a H2 gas in JP 2011-36988 A, substrate components are excessively diffused to the first layer, resulting in poor chipping resistance.
JP 2008-87150 A discloses a coated cutting tool comprising a TiN film; a columnar crystal TiCN film having an average crystal diameter of 0.05-0.5 μm when measured in parallel with the substrate surface, and an X-ray diffraction peak of a (422) plane in a 2θ range of 121.5-122.6° (CuKα); an aluminum oxide film; and a TiN film successively formed on a WC-based cemented carbide substrate. In the columnar crystal TiCN film, C/(C+N) is 0.70-0.90. Because the TiN film is formed directly on the substrate in JP 2008-87150 A, substrate components are largely diffused to the first layer, resulting in poor chipping resistance. Further, because the starting material gas for the columnar crystal TiCN film contains a TiCl4 gas, an CH3CN gas, a H2 gas, and an C2H6 gas, without a N2 gas, fine, columnar TiCN crystal grains do not uniformly grow, but coarse TiCN crystal grains are formed, resulting in poor chipping resistance.
Japanese Patent 4720283 discloses a coated cutting tool comprising a modified titanium carbonitride layer vapor-deposited on a WC-based cemented carbide tool substrate. The modified titanium carbonitride layer comprises a conventional TiCN layer formed on a TiCN film having an average thickness of 0.02-0.5 μm. The starting material gas for the TiCN film comprises TiCl4, C3H6, N2 and H2. When a conventional TiCN layer (thickness: 1 μm) as the first layer and a TiCN film as the second layer were formed directly on the WC-based cemented carbide tool substrate C, the coated cutting tool has a short tool life, because the conventional TiCN film has poor wear resistance and low adhesion to the substrate.
Japanese Patent 4534790 discloses a cutting tool comprising lower and upper layers formed on a substrate, the lower layer being a Ti compound layer (comprising a modified TiCN layer) having an average thickness of 3-20 μm, and the upper layer being a chemical-vapor-deposited aluminum oxide layer having an average thickness of 1-15 μm. The modified TiCN layer is formed by a usual chemical vapor deposition apparatus, using a starting material gas comprising TiCl4, CH3CN, C2H4, N2 and H2, under the conditions of a substrate temperature of 700-750° C. and a pressure of 25-40 kPa. However, the inventors' investigation has revealed that because Japanese Patent 4534790 uses C2H4 having remarkably higher reactivity to a WC-based cemented carbide substrate than the starting material gas (C2H6) in the present invention, W is excessively diffused from the substrate, resulting in poor chipping resistance. It has also been found that because the starting material gas pressure is 25-40 kPa, higher than the starting material gas pressure (5-10 kPa) for the TiCN layer in the present invention, fine columnar TiCN crystal grains do not uniformly grow, but coarse TiCN crystal grains are formed, resulting in poor chipping resistance.